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The present invention relates generally to high efficiency conversion of DC power to RF power and specifically to high efficiency DC to RF conversion by the use of active harmonic insertion.
High efficiency amplifier devices operate by amplifying an input signal to produce an output signal and in the process efficiently convert DC power to RF power. Conventionally, efficiency improvements have been achieved by operating amplifiers in the non-linear region, utilizing amplifiers such as class F amplifiers. Class F amplifiers generally rely on a passive network at the load and/or source to create resonance at particular harmonics of the fundamental frequency of the output signal. Such passive networks are well known in the art, and voluminous references detail their various designs. However, reliance on passive networks to create resonance at particular harmonics has inherent limitations. These include the potential lack of sufficient energy at particular harmonics and difficulty in controlling the relative phases of the harmonics with respect to the phase of the fundamental frequency. In addition, passive networks are generally susceptible to manufacturing variances of components, which can cause imprecision in the resonant characteristics that further degrade the efficient performance of the amplifier device.
Some methods have been proposed for modifying harmonic passive network amplifier devices to improve efficiency. One method in particular involves increasing the Power Gain (GP) of the amplifier device in order to increase its Power Added Efficiency (PAE). The relationship between GP and PAE is defined according to the following equation:   PAE  =      DE    ⁡          (              1        -                  1                      G            p                              )      
where DE is the Drain Efficiency of the amplifier device. As can be seen by the relationship above, an increase in GP relates to an increase in PAE (given that DE is held constant). Accordingly, efficiency of the amplifier device, at least in terms of its PAE performance, can be increased by simply increasing GP. One known method for increasing GP in this context involves the use of a pre-amplifier to pre-shape the input signal to a power amplifier such that the root mean squared (RMS) value of the input signal is increased. An increase in the RMS value of the input signal generally results in an increase in the power delivered to the load, which in turn increases GP. These methods are illustrated by U.S. Pat. No. 6,249,183 issued to Bosch et al., Bernhard Ingruber et al., High Efficiency Harmonic Control Amplifier, IEEE MTT-S International Symposium on Microwaves (1996), and Sachihiro Toyoda, High Efficiency Single and Push-Pull Power Amplifiers, IEEE MTT-S International Microwave Symposium Digest (1993). Although GP can be increased in this manner, the use of a pre-amplifier to pre-shape the input signal to increase its RMS value restricts the dynamic range of the input signal over which the amplifier device can operate. As the signal level of the input signal changes, the input-output characteristics of the pre-amplifier varies, and consequently the intended pre-shaping of the input signal cannot be maintained.
One method has been proposed as an alternative to relying on harmonic passive networks for generation of harmonics at the output of the amplifier device. The method involves a power amplifier device that inserts harmonic signals at the output of a power amplifier, the harmonic signals being generated by sampling, frequency multiplying, amplifying, and phase-shifting the input signal to the power amplifier. By actively inserting harmonics to the output of the power amplifier, instead of relying on passive harmonic networks, this method avoids the inherent disadvantages associated with passive harmonic networks such as harmonic energy deficiency and variances in manufacturability. This method is illustrated by U.S. Pat. No. 5,172,072 issued to Willems et al. However, in this method, active insertion of harmonic signals at the output of the power amplifier requires higher power to drive the harmonics, since signal levels are considerably higher at the output of the power amplifier. In applications requiring high efficiency operation of amplifier device, such as wireless and other limited-power devices, the additional requirement to provide such high power may indeed be impracticable. Moreover, significant insertion loss may occur at the point where the harmonics are inserted into the output of the power amplifier.
High efficiency DC to RF conversion with use of active harmonic insertion is provided for power amplification over a wide dynamic range of input signal level. Specifically, a power amplifier device including at least a final amplification stage is operated to receive an input signal of a fundamental frequency. A drive signal is produced which includes a fundamental signal component of the fundamental frequency and at least one harmonic signal component of a harmonic frequency that is substantially an integer multiple of the fundamental frequency, wherein relative phase shift and relative amplitude of the components are controlled over at least an order of magnitude of dynamic range of the input signal. As the signal level of the input signal decreases (or increases), the desired proportion of signal levels is maintained between the components. The drive signal is provided to the power amplifier device, and in response to the drive signal, an amplified output signal is produced at the final amplification stage.
In a specific embodiment, a voltage controlled oscillator (VCO) is operated at substantially the harmonic frequency to produce a harmonic signal corresponding to the harmonic signal component, and the harmonic signal and a signal corresponding to the fundamental signal component are combined to produce the drive signal.
In another specific embodiment, a digital synthesizer is operated to synthesize the drive signal.
The invention will be better understood by reference to the following description in connection with the accompanying drawings.